1. Field of the Invention
The present invention relates to a glycol dispersion of precipitated calcium carbonate. More particularly, it relates to a dispersion in glycol such as ethylene glycol of precipitated calcium carbonate for use as a material in the manufacture of polyester which is used as material especially for films or fibers, for the purpose of improving the polyester's friction coefficient, wherein the glycol dispersion of precipitated calcium carbonate comprises a grade of calcium carbonate having a good dispersibility in glycol and a good uniformity of the particle size and glycol such as ethylene glycol, propylene glycol and butylene glycol.
2. Description of the Prior Art
Polyester manufactured today commercially, polyethylene terephthalate (hereinafter called "PET") in particular, has excellent physical and chemical properties, and is being widely used for the manufacture of fibers, films and other moldings.
Meanwhile, however, it is known that it lacks in sliding property in the process of molding, further processing and handling of manufactured articles, this interferes with the workability and results in lowering the commercial values of the products. And, these troubles are mostly attributable to the high frictional coefficient of polyester itself. To overcome such problems, there have been proposed to date many methods which essentially consist in incorporating fine particles in polyester and imparting a proper degree of roughness to the surface of a molded article to thereby improve the surface sliding property thereof. As such fine particles, it is well-known to use fine particles of inorganic compounds insoluble in and inert to polyester such as titanium dioxide, silica, talc, kaolin and calcium carbonate added to the polymerization step in the form of a glycol slurry for the purpose of improving dispersibility thereof in polymer.
However, it is also known that these inorganic particles are generally insufficient in the affinity with glycol or polyester which leads to a defect of agglomerating in the slurry or during the polymerization. The presence of agglomerated coarse particles in polyester can cause yarn breakage in the spinning process, formation of the so-called "fish eye" et cetera and, in particular, drop-out or lowering of S/N ratio when it is used for manufacture of film for magnetic tape, and there has been a strong need for development of fine particles free from formation of coarse particles.
Among the fine particles used in polyester, calcium carbonate is widely used as filler in various fields such as paper, paints, rubbers and plastics. The calcium carbonates are roughly classified into two types in general, namely ground calcium carbonate and precipitated calcium carbonate (synthetic calcium carbonate).
The ground calcium carbonate is manufactured by mechanically pulverizing limestone and by subsequently classifying the resulting powder into various grades. This has an advantage of being relatively low in manufacturing cost, but has defects of being broad in particle size distribution and a powder fineness beyond a certain limit being unattainable by the present level of pulverizing and classifying technique.
The ground calcium carbonate is frequently used in polyester because of its affinity with polyester is better than other fine particles and it has hitherto been used in the following manners;
(1) A method wherein a commercially available grade of ground calcium carbonate or ground calcium carbonate was surface-treated with fatty acid, resin acid or alkali metal salts thereof, air classified repeatedly for elimination of coarse particles not less than 5 .mu.m in particle size, and then dispersed in glycol.
(2) A method wherein a commercially available grade of ground calcium carbonte was dispersed in glycol and then, after being subjected to wet-grinding by a wet-grinder such as sand mill, it is wet-classified for elimination of coarse particles not less than approximately 3 .mu.m in particle size.
The use of the ground calcium carbonate, notwithstanding, prepared by the processes as mentioned above in polyester involves the following drawbacks;
In case of (1);
(a) Even if a particularly fine grade of commercially available ground calcium carbonate is chosen and air classified, its particle size distribution before classification is very broad, having coarse particles of approximately 4-6 .mu.m in size contained therein as shown in FIG. 8. Even if a commercially available air classifier of the highest performance is used and classification is done repeatedly, it is difficult to perfectly eliminate coarse particles up to 3 .mu.m in size. For this reason, it is difficult to use the ground calcium carbonate prepared by this process in extremely thin polyester film for use in audio tapes, etc. An electron microscopic picture (.times.1,000) of a particularly fine grade of commercially available ground calcium carbonate (Super #2300, manufactured by Maruo Calcium Co., Ltd) is shown in FIG. 8.
(b) When ground calcium carbonate is surface-treated with fatty acid, resin acid or an alkali metal salt thereof for improving air classification efficiency, dispersion stability in glycol is adversely affected due to poor compatibility between such surface-treating agents and glycol.
(c) Since there is a limit to the fineness of commercially available grades of ground calcium carbonate, preparation of calcium carbonate of the desired particle size is impossible even by most elaborate air classification.
In case of (2);
(a) Since ground calcium carbonate as material is attritionally ground by the use of a wet-type grinder, it is relatively easy to obtain calcium carbonate of the desired average particle size compared with any of the methods described above under (1) but, since the mode of grinding is attritional, an increased amount of particles of unnecessary fineness results to broaden the particle size distribution. This means undesirable decrease of the absolute quantity of calcium carbonate contributing to improvement of frictional properties of polyester, which is a major object of the present invention, and, worse, such ultrafine particles tend to reagglomerate in glycol to form coarse secondary particles, thus possibly deteriorating the physical properties of polyester films or polyester fibers.
(b) Even when wet-grinding is performed with ground calcium carbonate, it is possible that part of the coarse particles, 4-6 .mu.m in size, contained in the material ground calcium carbonate remain in the calcium carbonate after grinding due to a phenomenon called "short pass" (a phenomenon wherein coarse particles in the material to be ground are discharged from a wet-type grinder substantially unground) and even if an attempt is made to eliminate such coarse particles by the use of a wet-type centrifugal classifier, the size of particles which can be eliminated in an economically acceptable manner is only reduced to approximately 1 .mu.m. Accordingly, the ground calcium carbonate prepared in such processes as aforesaid is not applied to a field where the perfect removal of the coarse particles exceeding 1 .mu.m is necessary like an 8 mm video tape.
For the foregoing reasons, when calcium carbonate prepared by a commercially available ground calcium carbonate is applied to polyester, it is difficult to improve the slidability of a polyester film or fiber more than the present level. And the improvement of this slidability influences running stability of audio tapes etc., and the running stability influences delicate sound properties required of such high grade tape. Hence calcium carbonate satisfying the following properties is urgently required by polyester makers, electric machine makers and the like, but to date calcium carbonate having such properties, as listed below, which would satisfy the need of the industry has not yet been proposed;
(i) calcium carbonate excellent in dispersion stability in glycol, PA0 (ii) calcium carbonate uniform in particle size, PA0 (iii) calcium carbonate not containing unnecessary particles such as coarse or ultrafine particles and being sharp in particle size distribution in glycol, PA0 (iv) calcium carbonate whose size is freely selectable.